In the late 1920s. The Oxford English Dictionary finds the first use of "101" as an introductory course number in a 1929 University of Buffalo course catalog. Colleges and universities began to switch to a three-digit course-numbering system around this time. In 1935, two researchers from Kent State published a paper celebrating the efficiency of the new system: "Recently college catalogs have revealed a commendable trend toward a logical arrangement of course numbers," they wrote. "The loose hodgepodge of former years is giving way to systematic arrangement."

The three-digit arrangement at Kent State began with a number corresponding to the college year. Freshman courses, for example, started with "1." The second digit referred to the content area, or to whether the course could be taken for credit, and the third described its place in a sequence of classes beginning with "0." An intro English class might have gotten the number "160," as a first-year class (1--) serving as the first in a sequence (--0) for a given subject (say, -6-).

Schools made up their own numbering systems, so Kent State's "160" might match up to the University of Buffalo's "101." But late adopters had an incentive to mimic the choices made by other schools. In the 1920s, students began to think of college as a means of getting a job, which meant they had to obtain a credential that could be compared to the credentials from other schools. It was easier to compare intro classes at several schools if they all had the same number.

The move toward standardizing (and numbering) course catalogs began in the late 19th century. The then-president of Harvard, Charles W. Eliot, started the trend when he introduced the elective system and redesigned the university's catalog in 1870-71. Until that point, a college curriculum comprised a set of mandatory courses without departmental or subject headings. As research areas became more specialized, other universities followed Harvard's lead and began to divide up their catalogs into numbered, departmental offerings.

Somewhere along the line, "101" migrated out of academic institutional jargon and into popular slang. (See the headlines listed in the "Related in Slate" section at the bottom of this column.) The OED finds an example of this "extended use" from 1986. Etymologist and Slate contributor Benjamin Zimmer cites a couple of earlier examples: A 1972 Time article jokes that "Social Relevance 101" must be "a basic course on TV these days." And in a stand-up routine from the early 1960s, Woody Allen jokes, "I took all the abstract philosophy courses in college, like truth and beauty, advanced truth and beauty, intermediate truth, introduction to God, Death 101."